stack: Replace "o" output with "r" input constraint
authorNick Desaulniers <ndesaulniers@google.com>
Mon, 19 Apr 2021 23:17:41 +0000 (16:17 -0700)
committerThomas Gleixner <tglx@linutronix.de>
Tue, 11 May 2021 07:56:11 +0000 (09:56 +0200)
commit2515dd6ce8e545b0b2eece84920048ef9ed846c4
tree3d6391ab0cdb68924c871a8e37b467683953f694
parent6efb943b8616ec53a5e444193dccf1af9ad627b5
stack: Replace "o" output with "r" input constraint

"o" isn't a common asm() constraint to use; it triggers an assertion in
assert-enabled builds of LLVM that it's not recognized when targeting
aarch64 (though it appears to fall back to "m"). It's fixed in LLVM 13 now,
but there isn't really a good reason to use "o" in particular here. To
avoid causing build issues for those using assert-enabled builds of earlier
LLVM versions, the constraint needs changing.

Instead, if the point is to retain the __builtin_alloca(), make ptr appear
to "escape" via being an input to an empty inline asm block. This is
preferable anyways, since otherwise this looks like a dead store.

While the use of "r" was considered in

  https://lore.kernel.org/lkml/202104011447.2E7F543@keescook/

it was only tested as an output (which looks like a dead store, and wasn't
sufficient).

Use "r" as an input constraint instead, which behaves correctly across
compilers and architectures.

Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://reviews.llvm.org/D100412
Link: https://bugs.llvm.org/show_bug.cgi?id=49956
Link: https://lore.kernel.org/r/20210419231741.4084415-1-keescook@chromium.org
include/linux/randomize_kstack.h